Calculations

Largest force comparison- 15,000 in lb or 173 kg m. (1730 Nm) for the 45.6 Cu In Hydraulic Motor that we use in LifeTrac

Shredding Rate

Precious plastic shredder - what is shredding rate of precious plastic shredder? 3kw, 50kg/hr. [1]
By comparison, we have 5W steppers. Minebea_Stepper_Motor_Specs
This allows us for 3000/5=600x less throughput. This means that for 12 hour run time, we get 1 kg of shred.
This is minimal, but doable for production or 2 filament spools/day. This means. $1200 revenue/month if selling spools at $20/spool.
This is a mere 5W. Use 100W - and get 10x-20x the production rate - of 20-40 spools/day. That is a potential business.

1" shafts made of mild steel would break at 3000 ft lb of torque (ref?)
Minimum torque requirement 200 ft lb, or 28 kg m. This is 1/6 the strongest motor.
Precious Plastic Shredder calls for 3 kg m from Lyman_Filament_Maker_Calculations#More. But that is old - and apparently not enough. The PP Pro shredder calls for 100 kg-m torque.

Motor Torque Calculator says about 1 kg m for 3000 rpm with 3kW drive as in Treadmill Motor. Compare to 173 kg m for strongest torque relevant to tractor drive as in 15000 In Lb Motors.
Our goal is in between for now, 30 kg m. Reduction of only 30x to 100 RPM.

Try a treadmill motor (3kW) and a drill motor (100W).

Brushed motor is 75-80% efficient. [2]
We can try a small DC motor, 30W. This yields a theoretical 12 kg per 12 hour day grind rate, which is just about correct. [3]
Ex. JYCRS390H - [4]. Not 30W. Appears to be 5W - [5]